The invention relates to method and apparatus for acceleration limiting a gas turbine engine and is more particularly directed to generating an acceleration limit as a nondimensional limit function independently of a pressure measurement.
The closed loop fuel control of gas turbine engines based on acceleration has become successfully implemented in various systems. These systems act directly to control the rate of change of the engine speed as a function of an acceleration term. The acceleration term is formed by differencing a scheduled term with an actual or an implied actual parameter of the gas generator. The actual acceleration of the engine is fed back through changes in the actual term for comparison with the scheduled term. The acceleration term, which after an integration effectively provides a datum for a proportional speed control loop, may, therefore, be a function of any of a number of control input parameters including engine speed, ambient pressure, temperature, compressor pressure, etc. Advantages of this form of acceleration control include consistent predictable accelerations which take account of fuel type, temperature, and altitude. The acceleration time is also generally independent of air bleed and power extraction status.
A closed loop fuel control for a gas turbine engine based on acceleration is more fully disclosed in U.S. patent application Ser. No. 210,938, filed in the name of Roland M. Evans on Nov. 28, 1980 (U.S. Pat. No. 4,423,592), which is commonly assigned with the present application. The disclosure of Evans is hereby expressly incorporated by reference herein. Other examples of closed loop systems of this type are illustrated in U.S. Pat. Nos. 4,018,044; 4,100,731; and 4,040,250.
The acceleration that is requested should be a function of the final desired output power or speed. Generally, it is desired that an acceleration be the safe maximum available from the engine for the particular operating conditions of the engine. Only the physical limitations of the engine should reduce the acceleration capability of the control. One set of acceleration limiting conditions is where the compressor begins to stall and the engine enters a surge condition destructive to the equipment. Therefore, the acceleration and fuel flow must be controlled so that a safe operating margin is maintained between stall conditions and the operating point so the engine does not encounter these problems. On most controls, an acceleration limiter, producing predetermined acceleration limits, is utilized to control the safety factor. When on acceleration limit, the fuel flow to the engine is scheduled by the acceleration limiter without regard to the steady running line of the engine.
Normally, this margin of safety is defined in terms of a surge line representing an acceleration limit which, if not exceeded, will help prevent the engine from stalling. Typically, the surge line has been defined as an open loop schedule in terms of parameter groups such as the fuel/air ratio of the engine (Wf/(Pc.sqroot.To)) versus the gas generator speed (N/.sqroot.To). When defined in this manner, the parameter groups are "nondimensional" (lacking only a geometrical scaling factor) and generally apply to all engines of the particular type for which the stall line was determined. A new schedule does not have to be developed every time an engine dimension is modified and is equally adaptable to all engines regardless of size of that design. The new schedules are merely scaled versions of the previous schedules based on the engine size changes.
For closed loop controls based upon acceleration, another parameter group that is frequently used to limit acceleration is (dN/dt)/Po where dN/dt is the acceleration limit of the engine, and Po the ambient pressure. While reducing the accuracy requirements somewhat as compared to an open-loop scheduling system, this method still requires the measurement of pressure. This measurement is comparatively expensive and unreliable to transduce electronically to the required accuracy over the environmental envelope of an aircraft gas turbine engine. Therefore, it would be highly desirable to provide a method and apparatus generating a scheduled acceleration limit for a closed loop fuel control system without the measurement of a pressure. It would, additionally, be highly desirable to generate the acceleration limit as a group of parameters that form a "nondimensional" ratio.